I think you are attributing more influence to Wilhelm II in 1914 than he really had. He was left out of most of the decision making during the crisis of July 1914.
Left out? Not by a long shot.
On July 4th, Wilhelm
personally ordered the German ambassador in Vienna to stop advising restraint calling the idea of restraint "nonsense". He then had the ambassador tell Vienna
"Germany would support the Monarchy through thick and thin, whatever action it decided to take against Serbia. The sooner Austria-Hungary struck, the better."
This is the infamous "Blank Check".
The next day on the 5th, Wilhelm repeats all of this again to the A-H ambassador in Berlin.
On the same day, the Chancellor, Foreign Ministry’s State Secretary, Minister of War, head of the German Imperial Military Cabinet, Adjutant General, head of the Naval General Staff, and Naval State Secretariat all meet in conference and endorse Wilhelm's blank check as Germany's best policy.
As others in this thread have pointed out, Austria would have never gone to war if Germany hadn't supported her and Germany, first in the person of Wilhelm on the 4th and 5th and quickly endorsed by the government on 5th, let Austria know in the strongest possible terms she had Germany's full support.
He could have put the brakes on war if his chancellor and the Army General Staff would have let him.
That was later in the month and he wasn't trying to stop the war. He was trying to impose an "East First" policy. The government and army told him that, due to the years of planning in place, such a change was impossible. (Although some General Staff drone wrote a book in the 1920s showing that it could have been possible.)
Whether Wilhelm had the power to stop the march to war is debatable and I don't believe he had the power. Because of his personal actions on July 4th and 5th, actions his government
later agreed to support, Wilhelm was very much responsible for starting the march to war very early in the crisis.